Treatment of high modulus graphite fibers to improve their bonding characteristics

ABSTRACT

A method of improving the bonding characteristics of high modulus graphite fibers is provided that comprises treating the fibers with a solution of osmium or ruthenium tetroxide and sodium iodate in water, dioxane or aqueous dioxane; washing and thereafter drying the treated fibers; contacting the treated fibers with a solution of tin tetrachloride in 2-butanone; and heating the fibers to remove the 2-butanone solvent. When graphite fibers so treated are used as a reinforcing material for epoxy resins, a composite is obtained that has a greatly improved interlaminar shear strength.

United States Patent Dauksys Apr. 2, 1974 [54] TREATMENT OF HIGH MODULUS3,627,570 12/1971 Cass et a1 117/169 R X GRAPHITE FIBERS o IMPROVE THEIR3,627,571 12/1971 Cass et al 117/228 X 3,635,675 1/1972 Ezekiel 264/29 XBONDING CHARACTERISTICS 3,720,536 3/1973 Scola et a1. 117/228 X Appl.No.: 250,715

US. Cl. 117/47 R, 117/62, 117/169 R Int. Cl B44d 1/14, B44d l/092 Fieldof Search ll7/47 R, 228, 169 R, 62,

117/118, DIG. 11

References Cited UNITED STATES PATENTS 4/1971 Hawkins et a]. ll7/47Primary Examiner-William D. Martin Assistant ExaminerStuart D. Frenkel[5 7 ABSTRACT A method of improving the bonding characteristics of highmodulus graphite fibers is provided that comprises treating the fiberswith a solution of osmium or ruthenium tetroxide and sodium iodate inwater, dioxane or aqueous dioxane; washing and thereafter drying thetreated fibers; contacting the treated fibers with a solution of tintetrachloride in Z-butanone; and heating the fibers to remove the2-butanone solvent. When graphite fibers so treated are used as areinforcing material for epoxy resins, a composite is obtained that hasa greatly improved interlaminar shear strength.

5 Claims, N0 Drawings TREATMENT OF HIGH MODULUS GRAPHITE FIBERS TOIMPROVE THEIR BONDING CHARACTERISTICS FIELD OF THE INVENTION Thisinvention relates to a method for treating high modulus graphite fibersso as to improve the bonding of the fibers to resinous materials. In oneaspect it relates to composites in which the treated fibers function asa reinforcing material for epoxy resins.

BACKGROUND OF THE INVENTION It is well known to use various reinforcingagents or fillers, such as carbon black, asbestos, glass fibers, silica,aluminum silicate and the like, to impart desired properties toelastomeric or resinous polymers. In recent years carbon and graphitefibers have been utilized extensively as reinforcing materials. Andbecause of their very high modulus of elasticity, graphite fibers haveproven to be very useful in fabricating structural composites that maybe subjected to great stressas in aircraft and aerospace vehicles.

Epoxy resins or polyepoxides have been used as the matrix for graphitefibers in fabricating composites or laminates to be used, e.g., asstructural members. However, a problem encountered with using epoxyresins as the matrix resides in the difficulty in establishing a strongbond or coupling between the fibers and the resin. Attempts have beenmade to overcome this problem by modifying the fibers, but anyimprovement in shear strength of the product has been accompanied by adegradation of other properties, e.g., tensile strength, by 25 percentor more.

It is an object of this invention therefore, to provide a method formodifying graphite fibers so as to improve their bonding relationshipwith epoxy resins.

A further object of the invention is to provide treated graphite fiberswhich, when used as a reinforcing material for epoxy resins, result incomposites having an improved interlaminar shear strength.

Another object of the invention is to provide a composite comprising anepoxy resin matrix having a high.

shear strength.

Still another object of the invention is to provide a method-ofpreparing a composite comprising an epoxy resin matrix.

Other objects and advantages of the invention will become apparent tothose skilled in the art upon consideration of the accompanyingdisclosure.

SUMMARY OF THE INVENTION The instant invention lies in the discovery ofa method for treating high modulus graphite fibers whereby theircapability to bond to or couple with epoxy resins is improved. Themethod comprises the steps of immersing the fibers in a solution ofosmium or ruthenium tetroxide and sodium idoate in water, aqueousdioxane or dioxane; water washing and drying the fibers after removalfrom the solution; contacting the dried fibers with a solution of tintetrachloride (SnCl and after removal from the latter solutionevaporating the solvent from the fibers. The use of graphite fiberstreated by this method in forming composites with epoxy resins resultsin products having an improved interlaminar shear strength with aminimal effect on other mechanical properties.

The graphite fibers used in the process are readily available fromcommercial sources. As precursors, a variety of polymeric or resinousmaterials can be utilized such as cellulosic fibers, includingregenerated cellulose or rayon yarns; polyacrylonitrile; copolymers ofacrylonitrile and a minor amount of, e.g., methyl acrylate, vinyl,acetate, styrene, vinyl toluene, vinylidene chloride, vinyl methylphthalate, ethylene, and the like;

.polybenzimidazoles, such as poly-2,2'-(m-phenylene)-5,5(dibenzimidazole; aromatic polyamides; aromatic polyimides; petroleumpitch; and the like.

The graphite fibers are often prepared by a three-step process,involving preoxidation at 200 to 350 C followed by carbonizationaccomplished by heating the oxidized fibers in an inert atomsphere at1,000 to 1,500 C. Thereafter, the carbonized fibers are heated in aninert atmosphere at a temperature ranging, e.g., from l,800 to 3,000 Cto provide graphite fibers. A method for preparing graphite fibershaving a very high modulus of elasticity is disclosed in US. Pat. No.

The epoxy resins with which the treated graphite fibers are employed arewell known and are readily available from commercial sources. In PolymerProcesses, lnterscience Publishers, Inc, pages 429-474 and 506-509(1965), epoxy resins are described by C. E. Schildknecht. Thepolyepoxides, which are characterizedby the presence of oxirane endgroups, may be prepared, as described in the aforementioned publication,by the reaction between phenolic compounds and epichlorohydrin. Epoxyresins are also disclosed in numerous patents, including US. Pat. Nos.2,643,239; 2,694,695; 3,098,056; and 3,403,13lfCuring of thepolyepoxides, which are usually in the form of a viscous liquid, may beaccomplished by mixing and heating the polymer with from about 1 to 30weight percent of a hardener or curing agent. A solvent may also beincluded in the system in order to control the viscosity of thepolyepoxide. In the normal curing operation, crosslinking occurs as aresult of the interaction of the terminal epoxy groups, the curing agentand some of the pendant hydroxyl groups.

A large number of the commercially available resins are prepared byreacting epichlorohdyrin and Bispheno] A [2,2-bis(4-hydroxyphenyl)propane]. Another example of an epoxy-type resin that is extensivelyused is epoxy-novolak resins prepared from the reaction of novolaks(phenol-formaldehyde resins) to attach epoxy groups to the polymerchain. The suppliers of epoxy resins generally furnish the hardeningagent for the particular resin, together with information as to amountto use, and the cure temperature and time. Examples of suitablehardening agents include polycarboxylic acid anhydrides, such asphthalic anhydride, pyromellitic anhydride, and maleic anhydride, whichmay be used with an amine accelerator, such as N-benzyldimethylamine or2,4,6-tris(dimethylaminomethyl)phenol.

From the foregoing discussion of graphite fibers and epoxy resins, it isseen that such materials are well known in the art. Furthermore,information concerning the materials are readily available in theliterature and from manufacturers. The present invention is primarilyconcerned with the treatment of graphite fibers from any source and thecomposites prepared with the treated fibers using any epoxy resin as thematrix.

As mentioned hereinbefore, in the initial step of the method, thegraphite yarn is immersed in a solution of osmium or ruthenium tetroxideand sodium iodate in water, dioxane or aqueous dioxane. The amount ofwater and/or dioxane used as the solvent can vary from zero to 100volume percent water and from zero to 100 volume percent dioxane, basedon a total of 100 volume percent. However, it is often preferred toutilize a solvent containing in the range of 40 to 60 volume percentdioxane and 40 to 60 volume percent water. While the amounts of osmiumtetroxide and sodium iodate can vary within rather broad limits, it isusually preferred to employ from about 0.05 to 1.0 gram of osmium orruthenium tetroxide and from about 25 to 200 grams of sodium iodate perliter of solvent. In preparing the solution, the two materials can beseparately added to the solvent. However, in a preferred procedure, theosmium or ruthenium tetroxide is added to a small portion of dioxane,e.g., to 100 milliliters, after which it is mixed with the solvent. Thesodium iodate can be added to the solvent either before or afteraddition of the tetroxide. During the immersion step, the solution ismaintained from about ambient temperature to 90 C, preferably from about80 to 90 C. The immersion time can vary within rather broad limits,e.g., from about 1 minute to 8 hours and longersThe amount of solutionthat is actually used is that which is sufficient to provide goodcontact between the fibers and the solution. For example, 1 liter ofsolution for 0.5 pound of fibers gives satisfactory results.

After the immersion step is completed, the fibers are removed from thesolution and water washed. In the washing operation, it is preferred touse hot distilled water, e.g., water at a temperature ranging from about50 to 80 C. Thereafter, the washed fibers are dried, preferably in avacuum oven. The fibers remain in the oven for a time sufficient toensure that they are completely dried. This is usually accomplished infrom 30 minutes to 1 hour with the oven maintained at a temperatureranging from about 90 to 1 10 C.

The dried fibers are next brought into contact with a solution of tintetrachloride in a suitable solvent therefor. The amount of the stannicchloride employed is usually in the range of about 0.2 to 1.0 volumepercent of the solvent. Examples of solvents that can be used includeacetone, 2-butanone, dimethylsulfoxide, dimethylformamide, ethylalcohol, propyl alcohol, and the like. It is often preferred to employ aketone as the solvent. The contact of the fibers takes place with thesolution at about room temperature (25 C). The fibers are in contactwith the tin tetrachloride solution for only a relatively short periodof time, e.g., from about to 60 seconds after which the fibers areheated to a temperature sufficient to evaporate the solvent. Thetemperature used will, of course, depend upon the particular solventemployed in preparing the solution. Longer contact times, e.g., up to 30minutes, can be utilized, particularly in batch operations. The heatingstep can be advantageously conducted by continuously passing the fibersunder an infrared lamp although other well known driers can be used.Also, it is to be understood that the solvent can be evaporated merelyby allowing the fibers to stand at room temperature. After the fibershave been treated as described, they are now in a form that renders themparticularly useful as a reinforcing material in the fabrication ofcomposites, utilizing an epoxy resin as the matrix.

Reference to the following illustrative examples, which are notintendedto be unduly limitative of the invention, will provide a morecomprehensive understanding of the invention.

EXAMPLE I A series of runs was conducted in which graphite fibers weretreated in accordance with the method of this invention after which theywere used with an epoxy resin to prepare composites. A control run wasalso conducted in which the same graphite fibers, but untreated, and thesame epoxy resin were used in fabricating a composite.

The graphite fibers used were a product sold by Union Carbide under thetrademark Thornel 50. The fibers had been sized with water. The productwas in the form ofa 2-ply yarn, twisted 1.5 to 2 turns per inch, having720 filaments per ply. The yarn had a modulus of elasticity of 50 X 10psi and a tensile strength of 325 X 10 psi. The single filaments had adiameter of about 0.0003 inch and a density of about 1.5 g/cc. Thornel50 graphite fibers are prepared from a cellulosic precursor.

In carrying out the runs according to the present method, hereinafterdesignated as runs l-6, the graphite fibers were initially immersed in asolution of osmium tetroxide and sodium iodate in 150 mls of dioxane and200 mls of water. 30 grams of sodium iodate and 0.2125 gram of osmiumtetroxide were used. The yarn remained in the solution for a period of30 minutes with the temperature of the solution being maintained atabout 71 C. At the end of the immersion period, the yarn in each run wasremoved from solution, washed with hot, distilled water and dried for 45minutes at 106 C under a vacuum.

Upon completion of the above-described steps, the washed and driedfibers were pulled successively through a room temperature solutioncontaining 0.2 ml of tin tetrachloride in 50 mls of 2-butanone, under aninfrared lamp to evaporate the 2-butanone, and through a solution of anepoxy resin. After passage through the epoxy resin solution, the fiberswere wound on a cylindrical mandrel so as to form thereon a monolayertape. The epoxy resin system, which was one supplied by Union Carbide,had the formulation shown in Table I.

TABLE I Parts by weight Epoxy resin ERL2256" 100 Hardener ZZLOBZO" 27Z-Butanone (1) Union Carbide identification symbol.

After appropriate B-staging (exposure overnight to atmosphere at roomtemperature), the tape was cut to mold dimensions. 10 plies of the tapewere placed on top of one another in a steel mold and molded accordingto the manufacturers recommendations. Thus, the mold was first heated at180 F for 2 hours followed by heating at 300 F for 4 hours. The mold wascooled to room temperature between the heating cycles and was maintainedat psi during the cycles.

The composites were tested to determine their interlaminar shearstrength. The results of the tests are shown below in Table II.

TABLE II Run No. Shear strength, psi

Control 3600 The data in the foregoing table demonstrate that the shearstrengths of the composites prepared with graphite fibers treatedaccording to the method of this invention were substantially greater,e.g., about 35 percent greater, than that of the composite fabricatedwith untreated graphite fibers. The greater shear strengths obtainedindicate that there is a substantial improvement in the coupling orbonding of the treated graphite fibers to the epoxy resin. Although itis not intended to limit the invention to a particular theory, it isbelieved that hydroxyl functionality on the graphite fibers is increasedby reaction of the osmium tetroxide with carbon-to-carbon double bonds.(The reaction of the osmium tetroxide is regenerative so that it can bereused in subsequent solutions merely by adding additional sodiumiodate.) The hydroxyl groups react with the sodium iodate to form anincreased number of carbonyl groups which complex with the tintetrachloride. The subsequent reaction of the complexes, which are alsoincreased in number, with oxirane groups of the epoxy resin results inimproved coupling between the fibers and the epoxy resin matrix.

EXAMPLE I] A run is conducted in which the graphite fibers used are thesame as those described in Example I. The fibers, as received from themanufacturer wound on a spool, are immersed for 3 minutes in a solutioncontaining 0.065 gram of osmium tetroxide and 85.8 grams of sodiumiodate in one liter of solvent. The solvent, maintained at 65 C,consists of S72 mls of water and 428 mls of dioxane. Thereafter, thespool and dried yarn are immersed in a room temperature solution ofstannic chloride in Z-butanone (0.4'volume percent SnCl for a period of25 minutes. The fibers impregnated with the stannic chloride solutionare then wound directly onto a mandrel after passing through a vacuumoven to evaporate the Z-butanone solvent. The procedure described inExample I is then followed in preparing a composite having a shearstrength comparable to those of Example I which were prepared with thetreated fibers. The procedure described in this example has theadvantage of eliminating the intermediate steps of the filament windingoperation as described in Example I.

As will be evident to those skilled in the art, modifications of thepresent invention can be made in the light of the foregoing disclosurewithout departing from the spirit or scope of the invention.

1 claim:

1. A method of improving the bonding characteristics of graphite fiberswhich comprises the steps of:

a. immersing the fibers in a solution of osmium or ruthenium tetroxideand sodium iodate in water, dioxane or aqueous dioxane;

b. removing the fibers from the solution;

0. water washing and thereafter drying the removed fibers;

d. contacting the dried fibers with a solution of tin tetrachloride in asolvent therefor; and

e. evaporating the solvent from the fibers recovered from step (d).

2. The method according to claim 1 in which the solvent for osmium orruthenium tetroxide and sodium iodate contains in the range of 40 to 60volume percent dioxane and 40 to 60 volume percent water; the solutioncontains from about 0.05 to 1.0 gram of osmium or ruthenium tetroxideand from about 25 to 200 grams of sodium iodate per liter of solvent;and the solution of tin tetrachloride contains from about 0.2 to 1.0volume percent tin tetrachloride, based on the solvent.

3. The method according to claim 2 in which the solvent for the tintetrachloride is a ketone.

4. The method according to claim 3 in which the ketone is Z-butanone.

5. The method according to claim 2 in which the solution of osmium orruthenium tetroxide and sodium iodate is maintained at a temperatureranging from about room temperature to C; the graphite fibers areimmersed in the solution for a period ranging from about 1 minute to 8hours; and the dried fibers are contacted with the tin tetrachloridesolution at room temperature for a period ranging from about 15 secondsto 30 minutes. I

2. The method according to claim 1 in which the solvent for osmium orruthenium tetroxide and sodium iodate contains in the range of 40 to 60volume percent dioxane and 40 to 60 volume percent water; the solutioncontains from about 0.05 to 1.0 gram of osmium or ruthenium tetroxideand from about 25 to 200 grams of sodium iodate per liter of solvent;and the solution of tin tetrachloride contains from about 0.2 to 1.0volume percent tin tetrachloride, based on the solvent.
 3. The methodaccording to claim 2 in which the solvent for the tin tetrachloride is aketone.
 4. The method according to claim 3 in which the ketone is2-butanone.
 5. The method according to claim 2 in which the solution ofosmium or ruthenium tetroxide and sodium iodate is maintained at atemperature ranging from about room temperature to 90* C; the graphitefibers are immersed in the solution for a period ranging from about 1minute to 8 hours; and the dried fibers are contacted with the tintetrachloride solution at room temperature for a period ranging fromabout 15 seconds to 30 minutes.